Motors, Actuators and Solenoids for Aerospace and Space Mechanisms
Aerospace motors and actuators are bought to a manufacturer or program specification and proven by characterization and environmental testing. For NASA space mechanisms, NASA-STD-5017B sets design and test requirements, lubrication is treated as a controlled process, materials must pass ASTM E595 outgassing screening, and the old MIL-S-4040 solenoid specification is inactive for new design.
Updated
Types and where they are used
The motor type follows from what the mechanism has to do: run continuously, hold position without power, or move once. NASA-STD-5017B treats brushless, stepper and brush motors separately because each fails differently.
| Type | How it works | Typical aerospace use |
|---|---|---|
| Brushless DC (electronically commutated) | Electronics switch current between windings using rotor position feedback | Reaction wheels, pumps, fans, gimbals, thrust vector control actuators |
| Stepper motors | Rotor moves in fixed steps as the drive rotates the stator field; holds by detent torque | Antenna and solar array pointing, instrument mechanisms, valves |
| Brush DC motors | Mechanical commutation through brushes | Ground equipment and short life uses; limited in long vacuum life |
| Electromechanical actuators (EMA) | Motor plus gear train or screw producing linear or rotary output | Flight control surfaces, engine gimbals, deployment and latch mechanisms |
| Solenoids | Coil pulls or pushes a plunger, or rotates an armature, over a short stroke | Latches, valve pilots, locks and release devices |
| Gearheads | Planetary, spur or harmonic reduction stages | Torque multiplication on any of the above |
NASA-STD-5017B defines a mechanism broadly. It applies to space flight mechanisms whose operation is required for safety or mission success, may serve as guidance for ground support equipment, and counts valves, ordnance-actuated mechanical devices, quick release pins and actuated threaded interfaces as mechanisms.
The standards that govern them
Motors and actuators rarely have a military QPL. The governing documents are the program's mechanism, materials and environmental standards, plus a source control drawing for the part itself. Confirm the current revision on the issuing body's site before you invoke any of these.
| Document | What it covers | Status we read |
|---|---|---|
| NASA-STD-5017B (approved 6 Dec 2022) | Design and development requirements for mechanisms: torque margin, lubrication, motors, springs, gears, qualification, life and acceptance testing | Current on the NASA Technical Standards System; supersedes NASA-STD-5017A w/Change 1 |
| NASA-STD-6016 | Standard materials and processes requirements for spacecraft | Cited by NASA-STD-5017B as governing lubrication as a process |
| ASTM E595 | Total mass loss and collected volatile condensable materials from outgassing in a vacuum environment | Test method cited by NASA-STD-5017B and EEE-INST-002 |
| GSFC-STD-7000B (28 Apr 2021) | General Environmental Verification Standard (GEVS) for Goddard payloads, subsystems and components | Active on the NASA Technical Standards System |
| MIL-S-4040G (6 Nov 2012) | Electrical solenoids, axial and rotary stroke, general specification | Inactive for new design after 28 June 1996; no superseding specification |
Aircraft actuators add the environmental qualification of the equipment they sit in, usually RTCA DO-160 or MIL-STD-810; see circuit breakers and switches for how those two documents are scoped.
Have a part like this?Send it in under a minute. A person replies within 1 business day.Reading a solenoid classification
Motor and actuator part numbers are manufacturer catalog codes, and we do not decode them here because the coding differs by maker. Legacy solenoids are the exception: MIL-S-4040G classifies every solenoid with a Roman numeral that captures action, duty and rotation, and old drawings still carry these.
| Symbol | Type | Duty | Rotation |
|---|---|---|---|
| I | Pull | Continuous | N/A |
| II | Pull | Intermittent | N/A |
| III | Push | Continuous | N/A |
| IV | Push | Intermittent | N/A |
| V | Rotary | Continuous | Clockwise |
| VI | Rotary | Intermittent | Clockwise |
| VII | Rotary | Continuous | Counterclockwise |
| VIII | Rotary | Intermittent | Counterclockwise |
Duty is the line that causes trouble. An intermittent duty solenoid energized continuously will overheat, and the duty cycle on the manufacturer's sheet is a hard limit, not a guideline. Because the specification is inactive for new design with no replacement, a solenoid on a new drawing should be defined by a source control drawing with stroke, force, coil resistance, duty cycle, temperature range and environment written out.
Space mechanism considerations: lubrication, vacuum and outgassing
In vacuum, the lubricant and the materials decide whether a mechanism works for its full life. NASA-STD-5017B requires every surface in contact whose friction under relative motion hurts performance to be lubricated, and says dissimilar metals, though encouraged, are not a substitute for lubrication. It lists the factors lubricant selection shall consider, including:
- The service environments: ground storage, pressure, temperature, radiation, dust, humidity and salt spray.
- Creep, viscosity index and elastohydrodynamic film thickness of wet lubricants.
- Protection against galling and friction welding in boundary lubrication.
- Outgassing and breakdown products that could contaminate sensitive surfaces.
- Depletion of wet lubricants through evaporation, creep, consumption and fling, and wear-out and debris of dry lubricants.
- Compatibility with other lubricants, substrates, platings, coatings and preservative or shipping oils.
Outgassing. NASA-STD-5017B says NASA evaluates the volatility of materials, including wet lubricants, with ASTM E595. EEE-INST-002 states nonmetallic materials shall not exceed 1 percent total mass loss or 0.1 percent collected volatile condensable material under that test. Motor potting, wire insulation, magnet adhesives and gearhead seals all need data.
Creep and barrier films. The standard notes that perfluoropolyalkylether (PFPE) oils have surface tensions of 17 to 25 dyne/cm and multiply alkylated cyclopentane oils about 32 dyne/cm, while barrier films sit near 10 dyne/cm, which is why barrier films keep oil in place and off optics, brakes and clutches.
Brushes in vacuum. NASA-STD-5017B warns that terrestrial brush materials become abrasive without moisture, that even trace moisture in a ground test can give a falsely good life result, and that the brushed motor option should be discarded if long operating life in vacuum is required. Where brushes have worked in vacuum, the standard cites copper or silver combined with molybdenum disulfide and graphite. Dry film lubricants are covered in our dry film lubricants reference.
Motor characterization the standard requires
NASA-STD-5017B requires each flight motor to be characterized with measured data, and the lists are a ready made acceptance data sheet.
- Brushless motors: torque constant, motor constant, torque versus speed curve using flight-representative drive electronics, phase resistances, phase inductances and end-to-end commutated torque performance. The minimum torque must be verified with flight-representative electronics, because a dynamometer run will not reveal the torque holes where commutation switches windings.
- Stepper motors: powered breakaway torque, unpowered detent torque over a full rotation, pull-in and pull-out torque with representative inertia and step rates, step accuracy, detent to powered null alignment, and phase resistances and inductances, all with flight or flight-representative drive electronics.
- Brush motors: established maximum temperature limits for windings and materials, not exceeded in worst case qualification environments.
The drive electronics caveat matters to a buyer. The standard notes that a radiation hardened transistor can have gate capacitance up to an order of magnitude different from an off the shelf part, which changes switching and mechanism response, so ask what driver the supplier's test data was taken with.
Qualification, life and acceptance testing
Qualification proves the design; acceptance proves each unit. NASA-STD-5017B requires each flight mechanism to see environmental acceptance testing that exposes it to all environments it will experience in service, in each service configuration and the right operating or non-operating state. It also requires a run-in test on each mechanism before any other acceptance test, to screen early life defects and let performance settle before trends are measured. The standard recommends acceptance testing qualification units before their qualification program as well.
Life testing is a separate requirement in the same standard, with factors applied to the expected service cycles, and its appendix gives recommended qualification and acceptance test matrices. For Goddard programs, GSFC-STD-7000B describes the environmental verification baseline and test levels for payloads, subsystems and components. Where protoflight testing is used, NASA-STD-5017B defines it as a strategy with no dedicated qualification article, so flight hardware carries the qualification burden.
If you need chamber, vibration or thermal vacuum time for these tests, see space test lab time.
Documentation to require
A motor or actuator is only as good as its paperwork when something goes wrong in test. A complete delivery for flight or flight-like hardware includes:
- Source control or specification drawing with revision, and the manufacturer's part and serial numbers.
- Characterization data per the lists above, with the drive electronics identified.
- Acceptance test report including run-in data, and qualification or similarity rationale.
- Lubricant identification, lot, quantity and application method, since lubrication is a controlled process.
- Materials list with ASTM E595 outgassing data for nonmetallics.
- Material certifications for structural and magnetic materials, and DFARS specialty metals compliance where the contract invokes it; see DFARS specialty metals.
- Certificate of conformance tying serial numbers to the order, and any first article inspection; see AS9102 first article inspection.
What to put on the purchase order
- Manufacturer part number and the source control drawing or specification with revision.
- Performance requirements to verify: torque or force, speed, stroke, duty cycle, voltage, temperature range.
- Required characterization and acceptance tests, the drive electronics to use, and the data to deliver.
- Lubricant and lubrication process, with no substitution without written approval.
- Outgassing, cleanliness and contamination control requirements, and bagging for clean areas.
- Serialization and lot traceability of the motor and its critical internal parts such as bearings and magnets.
- Shelf life and storage limits for lubricated assemblies, and any required exercise before use.
- Packaging that protects shafts, connectors and leads, with desiccant or purge if specified.
- Counterfeit avoidance and quality flow-downs from the prime contract.
Counterfeit and quality risks, and how to receive them
The risks with motors are substitution and unknown history more than outright fakes. Watch for commercial motors relabeled with a space grade part number, a change of lubricant or bearing source without notice, surplus mechanisms with no record of storage or prior cycling, and gearheads rebuilt from mixed parts. A motor with integrated drive electronics or encoders contains electronic parts, which may bring DFARS counterfeit clauses into play; ask your contracts team. Our counterfeit parts avoidance guide covers the clauses.
At receiving:
- Match serial numbers on the unit, nameplate, test data and certificate.
- Check that test data was taken on this serial number, not a representative unit, unless the order allowed that.
- Rotate by hand where allowed and note roughness, cogging that differs from the detent data, or binding.
- Inspect leads, connectors, shafts and mounting faces for damage and contamination, and keep caps and bags on clean hardware.
- Confirm lubricant and lot records are in the package before the unit goes to stores.
See receiving inspection for the general routine.
How we source motors and actuators
We do not stock motors or actuators, publish prices or act as an authorized or franchised distributor for any manufacturer. We source the parts on your drawing through the manufacturer or its authorized and franchised distributors, flow your characterization, lubrication and data requirements down in writing, and check the delivered package against the PO before shipment.
- Motors, gearheads and solenoids bought alone or kitted with connectors, harnesses and mounting hardware.
- Parts combined with machined housings, brackets and test fixtures on one purchase order; see CNC machining.
- Out of production motors and legacy MIL-S-4040 solenoids chased under counterfeit-avoidance controls; see obsolete aerospace parts.
Related parts: aerospace bearings, greases and lubricants, sensors and transducers, relays and contactors, and the electromechanical overview.
Request motors, actuators or a mechanism kit
Send the part numbers or source control drawings, quantities, and the characterization, lubrication and test data you need through the harness and electromechanical quote form. Those requirements decide which manufacturers can quote. A person replies within one business day, and every order ships with a certificate of conformance. For controlled drawings, read how we handle files and controlled programs.
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Questions
Can brushed DC motors be used on spacecraft?
Sometimes, but with care. NASA-STD-5017B notes that brushed motors have flown successfully but have difficulty working consistently in vacuum, because terrestrial brush materials become abrasive without moisture. It says the brushed option should be discarded if long operating life in vacuum is required, and warns that trace moisture in a ground test can make a life test look falsely good.
What outgassing limits apply to motor materials?
For NASA work the common screen is ASTM E595. EEE-INST-002 states that nonmetallic materials shall not exceed 1 percent total mass loss or 0.1 percent collected volatile condensable material under that test, and NASA-STD-5017B says NASA uses ASTM E595 to evaluate wet lubricants as well. Potting, insulation, adhesives and seals inside the motor all need data or a bakeout plan.
What does NASA-STD-5017B require for stepper motor testing?
Measured powered breakaway torque, unpowered detent torque over a full rotation, pull-in and pull-out torque with representative inertia, friction and step rates, step accuracy, detent to powered null alignment, and phase resistances and inductances. Testing shall use flight or flight-representative drive electronics, and the rotor polar inertia shall be calculated.
Is MIL-S-4040 still used to buy solenoids?
Only for legacy parts. MIL-S-4040G, dated 6 November 2012, states it is inactive for new design after 28 June 1996 with no superseding specification. Existing drawings can still reference it for replacement parts, but a new design should define the solenoid on a source control drawing with stroke, force, duty cycle, coil data and environments written out.
Why does lubrication need its own process control?
Because performance depends on the lubricant chosen, its quantity, location and application method. NASA-STD-5017B treats mechanism lubrication as a process subject to NASA-STD-6016 and explains that the wrong parameters raise drag and heat and reduce torque margin, too much lubricant creates contamination and too little shortens life. Lock the lubricant and process on the order.
What is a run-in test and why is it required?
A run-in, or wear-in, test operates the mechanism before other acceptance testing. NASA-STD-5017B requires it on each mechanism to screen out material and workmanship defects that show up early in life and to let performance settle to steady state, so later trends and anomalies are easier to see. Ask for the run-in data in the delivered package.
Related
Sources
- NASA-STD-5017B, Design and Development Requirements for Mechanisms (approved 2022-12-06)
- NASA-STD-5017B full text PDF, NASA Technical Standards System
- GSFC-STD-7000B, General Environmental Verification Standard (GEVS), NASA Technical Standards System
- MIL-S-4040G, Solenoid, Electrical, General Specification (6 Nov 2012), EverySpec
- NASA/TP-2003-212242, EEE-INST-002, outgassing limits per ASTM E595
- NASA Outgassing Data for Selecting Spacecraft Materials (ASTM E595 test basis)
- Acquisition.gov, DFARS 252.246-7008 Sources of Electronic Parts
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